With the recent spread of portable electronic equipment, such as mobile phones, video cameras and notebook computers, the demand for high-capacity secondary batteries as a current source, especially nonaqueous secondary batteries utilizing the intercalation-deintercalation reaction of lithium ions, has been increasing. As the functions of the electronic equipment are improved, the demand for higher capacity of secondary batteries has also been desired. Packing an active material into a battery container as much as possible is one approach to achievement of high capacity. Along this line, it has been attempted to reduce elements other than an active material, for example, to make a reduction in thickness of a current collector and to displace the decrease with an active material. However, a current collector having a reduced thickness has reduced strength and tends to be cut on receipt of stress on rolling of an electrode sheet or during a charge. In particular, a positive electrode current collector of a nonaqueous secondary battery is generally made of aluminum foil having relatively low strength among metals and is liable to be cut when its thickness is reduced.
The inventors of the present invention have previously achieved an increase in capacity by using a specific metal oxide having high ability of lithium ion intercalation as a negative electrode active material. However, lithium ion intercalation is accompanied by expansion of the negative electrode active material, which imposes pressure on the positive electrode sheet, causing the sheet to be cut.
Various aluminum alloys (JP-A-2-204976, the term "JP-A" as used herein means an "unexamined published Japanese patent application") and high purity aluminum (JP-A-6-267542) have been proposed for use as a positive electrode current collector of a nonaqueous secondary battery, but these materials are insufficient in mechanical strength under the above situation.